The performance of semiconductor devices can be affected significantly by impurities present in the semiconductor materials. For example, in MOS devices, the leakage current is controlled primarily by impurity segregation/precipitation at the Si-oxide interface. In most microelectronic silicon devices, the quality of the as-grown material is quite high. Yet, when the device is fabricated, it can have a much higher concentration of impurities because of impurity in-diffusion during the device fabrication—from cleaning procedures, furnaces used for various processes, such as oxidation, contact formation, etc. In view of these problems, semiconductor device fabrication techniques include (as an insurance policy) processes that can remove impurities from the device. The removal of impurities from an active region of a device to a benign region is referred to as “impurity gettering”. Most microelectronic devices use only the surface region of a wafer. Therefore, impurities can be gettered from the surface region to materials located deeper inside of the wafer, i.e., farther away from the surface region, where they can reside without interfering with the operation of the device. This approach, called “internal gettering,” takes advantage of high concentrations of oxygen in a Czochralksi wafer. The oxygen is made to precipitate within the thickness of a wafer trapping impurities with them. These impurities are drawn from the surface of the wafer to produce a denuded zone at the surface of the wafer. Because, the microelectronic devices are typically surface devices, such a gettering technique is well-suited for microelectronics.
Impurity gettering is also extremely important for Si solar cells. However, in Si solar cells, the gettering is used to improve the quality of the as-grown material. The photovoltaic silicon (PV—Si) industry uses both single- and multi-crystalline (mc) wafers that are grown by techniques specially developed to produce low-cost material. Typically, the single crystal ingots are grown by a Czochralski (CZ)-type process, and mc-Si is either cast or in the ribbon form. Because the substrate cost must be kept low, the PV industry employs a host of cost-cutting measures that include low-quality poly feedstock, a lower degree of cleanliness and control in the crystal growth process, and a high crystal-growth rate. These cost-cutting measures can, and often do, compromise the crystallinity, as well as the chemical purity of the material. Concomitantly, PV-Si often has high concentrations of impurities and defects.
PV-Si manufacturers often use low-grade feedstock, such as pot-scrap, off-spec, and remelt silicon—much of which is rejected material from the microelectronic industry and contains impurities. The impurities present in the feedstock remain when the feedstock is melted, and they get carried with the melt into the crystalline ingots grown from the melt, as dictated by the segregation coefficients. Hence, in general, the PV starting material, i.e., the crystalline or multicrystalline Si ingots, will have high impurity content. Therefore, substrate wafers cut from such Si ingots will also have that high impurity content. Typically these substrates contain C and/or O in near-saturation levels, as well as transition metals in the range of 1012-1014 cm−3. Because of their high concentrations, in many cases impurities can and do precipitate at preferred sites, such as at extended defects, grain boundaries, and defect clusters. The chemical structure of such precipitates can be quite complex. For example, micro-X-ray analyses have shown that some precipitates are predominantly metallic but have significant amounts of oxygen and/or carbon associated with them. This may indicate that metal precipitates are silicides, carbides, and oxides. On the other hand, this may mean that metal precipitation is a secondary process that takes place in the proximity of pre-existing oxygen/carbon precipitates. Such a phenomenon may occur as a local stress relaxation mechanism.
The single-crystal CZ ingots for PV are pulled at growth rates that can be many times faster than that of the conventional growth for microelectronics. The fast cooling rates used in pulling CZ ingots are accompanied by excessive thermal stresses that lead to generation of defects in the crystalline structure. Consequently, the single crystal material is expected to have high concentrations of quenched-in, non-equilibrium, point defects. In some cases, a portion of the ingot may acquire a high density of crystal defects (primarily dislocations) and even lose the crystallinity and become multicrystalline (mc). Such mc-Si substrates typically have very large grains with small grain boundary areas that produce only small effects on the device performance. The dominant intragrain defect is dislocation. High-quality mc-Si substrates have a tendency to form clusters of defects. The average defect density is about 105 cm−2; however, there can be localized clusters of defects where the defect density can exceed 107 cm−2. Our previous work has shown that such defects include networks of dislocations, stacking faults, and grain boundaries. Detailed analyses have shown that such defect clusters are also sites of impurity precipitates. It is rather interesting and a matter of curiosity that impurity precipitation occurs at defect clusters rather than at grain boundaries or at other isolated defects.
Gettering for Solar Cells
It is well-known that the performance of solar cells would be quite poor if the devices had as high concentrations of impurities as in the as-grown PV-Si. Fortunately, some of the impurities are removed during the device processing. Solar cells, being minority-carrier devices, use nearly the entire bulk of the device. Hence, the internal gettering technique is not suitable for solar cells. It is more attractive to apply external gettering techniques to clean up the bulk of the material. In the external gettering processes, the impurities are drawn to the surface and trapped. Phosphorous diffusion and Al alloying are some of the processes that have worked well for efficient gettering of solar cells.
In Al gettering, the goal is to draw impurities from the Si crystal structure into the Al and trap them there, so they cannot migrate or diffuse back into the Si crystal. Typically, a thin layer of Al (about 1 μm) is deposited onto a surface of the Si wafer or a partially completed device comprising a Si wafer. The Si wafer or device is then heated to about 800 to 850° C. for 30-60 minutes. The Al is liquid in this temperature range. Most metallic impurities in the Si crystal material, particularly those that are responsible for degrading minority carrier lifetime, are highly soluble in the liquid Al. Therefore, it is believed that, during this thermal process, impurities in the Si crystal material diffuse toward the liquid Al, where they are dissolved in the Al layer. Upon cooling and resulting solidification of the Al, the impurities are expected to remain trapped in the solid Al or in a Si—Al alloy region that may be formed at the Si—Al interface during this thermal process. The result is that the bulk of the Si wafer is left with a greatly reduced impurity concentration, which increases the solar cell performance, primarily due to improvement in minority carrier diffusion length in the regions from which the impurities were removed during this gettering process.
Because these processes are used extensively in solar cell manufacturing for junction and contact formation, all Si solar cells experience a certain degree of gettering. However, it is often necessary to optimize each of these process steps such that the highest degree of gettering is attained without sacrificing the junction or the contact properties. For example, in solar cell fabrication, impurity gettering by Al occurs as a by-product during formation of back surface field. However, because high temperatures are involved in this process step, it should be used prior to low temperature process steps, such as formation of the electric contacts on the device. Consequently, Al gettering, when used as described above, is not particularly effective against impurities that diffuse into the Si in subsequent process steps, which will remain in the silicon material and adversely affect the device performance. It is also likely that the impurities gettered into the Al region can be detrapped during subsequent processing. Therefore, a preferred mode of gettering is to apply it as the last process step in fabricating a device in order to ensure removal of impurities present in the “as-grown” substrate as well as those that diffuse into the substrate during the device fabrication steps. However, the conventional gettering procedure described above is, unfortunately, not compatible with a finished device.
Furthermore, Al treatment at high temperatures (>500° C.) produces an interface that absorbs light strongly. The presence of such an absorbing interface in a solar cell is accompanied by a loss in the cell efficiency, because light that reaches such an interface will not be reflected back to the active semiconductor region where it can be absorbed and converted to electric energy.
The high temperatures and/or long processing times, i.e., large time×temperature product, are required in the state-of-the-art (prior to this invention) Al gettering, as currently used in solar cell fabrication processing, because of the slow diffusivity of impurities dissolved in the Si substrate. The slower the diffusivity of the impurities, the longer times and/or higher temperatures will be required for such impurities to diffuse through the Si substrate to the Al, where they can be trapped as described above.
However, diffusivity of impurities dissolved in the Si substrate is not the only problem that drives up time×temperature product. As explained above, many impurities precipitate at the defect clusters. To getter such precipitated impurities, the precipitates must be dissolved before the impurities can become mobile and diffuse through the Si. Unfortunately, impurity dissolution is a very slow process at reasonable process temperatures. Such a dissolution depends on the temperature as well as the precipitate size.
For example, calculations show that, for the Al gettering process at 800° C., it will take more than 7 hours to reduce the total Fe concentration by three orders. For larger precipitates, still longer gettering times are needed; e.g., for the precipitate size of 50 nm, it will take many days at 800° C. to notice any gettering effect. We expect that a similar situation holds also for Cr, but only more difficult, because of its lower (one order of magnitude) diffusivity value.
These discussions clearly show that gettering of defect clusters (that have precipitated impurities) can be very difficult, and that conventional processing techniques cannot dissolve the impurities within such defect regions. It is important to note that, while effective gettering can be achieved throughout most of the substrate material by such conventional processing steps, the local regions of defect clusters in the substrate material remain effectively unchanged. Therefore, even in conventional processes that are designed to achieve a significant reduction in the dissolved impurity concentrations in the Si substrate, thus some—even significant—improvement in device performance, the over-all improvement potential is curtailed considerably by a lack of gettering in the defected regions of the substrate. In a large-area device, the local regions of high recombination that occur at such defect clusters can lead to “shunts” that can severely degrade the voltage-related device parameters. Therefore, such regions can strongly limit the device performance. Unfortunately, dissolution of such impurities requires high temperatures and cool-down cycles that are not compatible with conventional semiconductor processing.
Al gettering is a very valuable method that allows commercial fabrication of high efficiency solar cells on low-cost silicon substrates. Experimentally, the Al alloying is done at 800-850° C. for about 30 minutes. This technique works reasonably well, but it has many disadvantages, such as:                (i) high time and temperature process;        (ii) back interface becomes nearly non-reflecting;        (iii) often the front contact and back contact formation requires separate processing; and        (iv) improvements in cell performance and efficiency are still curtailed by defect clusters in which impurities are precipitated and which can lead to shunts, especially in large-area devices, that can strongly limit device performance.        